Thermo-chemical surface treatments of iron and steel by means of nitrogen or carbon carrying gases are well-known processes, called nitriding or carburising, respectively. Nitrocarburising is a process in which a gas carrying both carbon and nitrogen is used. These processes are traditionally applied to improve the hardness and wear resistance of iron and low alloyed steel articles. The steel article is exposed to a carbon and/or nitrogen carrying gas at an elevated temperature for a period of time, whereby the gas decomposes and carbon and/or nitrogen atoms diffuse through the steel surface into the steel material. The outermost material close to the surface is transformed into a layer with improved hardness, and the thickness of this layer depends on the treatment temperature, the treatment time and the composition of the gas mixture.
U.S. Pat. No. 1,772,866 (Hirsch) discloses a process for nitriding an article of iron or molybdenum steel in a crucible with urea. The article and urea are introduced together in the crucible and then heated to a temperature sufficiently to release nascent nitrogen from urea.
Dunn et al. “Urea Process for Nitriding Steels”, Transactions of the A. S. M., page 776-791, September 1942, discloses a process for nitriding steels using urea. Urea was selected as a cheap material known to evolve ammonia upon heating and because it is easy to handle and store. In one arrangement solid urea is heated together with the steel article in a nitriding furnace. In another improved arrangement the urea was heated in an external generator and the evolved ammonia was supplied to a furnace containing the steel article.
Chen et al., Journal of Materials Science 24 (1989), 2833-2838, discloses nitrocarburising of cast irons by treatment with urea at 570° C. in 90 min. It is stated that urea dissociates at temperatures of between 500 and 600° C. into carbon monoxide nascent nitrogen and hydrogen.
Schaber et al., Thermochimica Acta 424 (2004) 131-142 (Elsevier) analysed the thermal decomposition of urea in an open vessel and found a number of different decomposition products including cyanic acid, cyanuric acid, ammelide, biuret, ammeline and melamine during the heating at temperatures from 133 to 350° C. Additionally, substantial amounts of NH3 are formed by the different decomposition sub-reactions. Substantial sublimation and formation of further decomposition products occurs at temperatures above 250° C.
Accordingly, during the decomposition of urea, it is not completely known which intermediate products occur and how long time each of them occurs before a further decomposition takes place when urea is heated to temperatures up to 500° C.
Cataldo et al. [Journal of Analytical and Applied Pyrolysis 87 (2010) 34-44] analysed the thermal decomposition of formamide (HCONH2). The reaction is rather complex and involves decomposition products as HCN, NH3 and CO.
In nitriding and nitrocarburising praxis the activation of the surface prior to actual treatment is often established by an oxidation treatment at a temperature ranging from, typically, 350° C. to just below the nitriding/nitrocarburising temperature. For highly alloyed self-passivating materials the pre-oxidation temperature is very high and appreciably higher than the temperature at which nitriding/nitrocarburising can be carried out without avoiding the development of alloying element nitrides. Various alternatives for the activation of self-passivating stainless steel have been proposed.
EP 0588458 (Tahara, et al.) discloses a method of nitriding austenitic steel comprising heating austenitic stainless steel in a fluorine- or fluoride-containing gas atmosphere for activation followed by heating the fluorinated austenitic stainless steel in a nitriding atmosphere at a temperature below 450° C. to form a nitrided layer in the surface layer of the austenitic stainless steel. In this two stage process the passive layer of the stainless steel surface is transformed into a fluorine-containing surface layer, which is permeable for nitrogen atoms in the subsequent nitriding stage. The fluorine- or fluoride-containing gas atmosphere itself does not provide nitriding of the stainless steel article. Addition of halogen- or halide-containing gases for activation is a general method and is known to behave aggressively towards the process equipment interior and can lead to severe pitting of the furnace, fixtures and armatures.
EP 1521861 (Somers, et al.) discloses a method of case-hardening a stainless steel article by means of gas including carbon and/or nitrogen, whereby carbon and/or nitro-gen atoms diffuse through the surface of the article, the case-hardening is carried out below a temperature at which carbides and/or nitrides are produced. The method includes activating the surface of the article, applying a top layer on the activated surface to prevent repassivation. The top layer includes metal which is catalytic to the de-composition of the gas.
WO2006136166 (Somers & Christiansen) discloses a method for low temperature carburising of an alloy with a chromium content of more than 10 wt. % in an atmosphere of unsaturated hydrocarbon gas. The unsaturated hydrocarbon gas effectively activates the surface by removal of the oxide layer and acts as a source of carbon for subsequent or simultaneous carburising. In the listed examples acetylene is used and the du-ration of the carburising treatment ranges from 14 hours to 72 hours. An inherent downside by applying unsaturated hydrocarbon gas as a carburising medium and as activator is the strong tendency for sooting, which effectively slows down the carburising process and prevents control of the carbon content in the steel. In order to sup-press the tendency for sooting the temperature has to be lowered, which results in even longer treatment times (cf. above).
EP1707646B1 discloses a method for activation of metal surface prior to nitriding or carburising. A carbon containing gas such as CO or acetylene and a nitrogen containing gas such as NH3 are introduced into a furnace and heated to at least 300° C. By reaction with a metallic catalyst HCN is formed. For sufficiently high concentrations of HCN (100 mg/m3) the passive surface of the metallic member is activated. The examples shown describe activation of stainless steel; the diffusion treatment is carried out at a temperature of 550° C., which results in the precipitation of nitrides or carbides. The temperature for the activation is stated to be above 300° C. for a sufficient reaction rate between the carbon bearing compound and NH3. This method therefore requires comparatively high temperatures needed for reacting the two gases.
JP2005232518A discloses a surface hardening treatment method in which a gaseous mixture comprising a carbon feeding compound and a nitrogen feeding compound, the mixture being gaseous at 150° C., is heated to above 200° C. A catalyst installed in the furnace converts the gaseous mixture to HCN which then acts on the surface of a metallic article to modify and activate a passivated film on the surface. Successively, gas nitriding and/or gas nitriding-carburising is performed at 400 to 600° C. This method requires the provision of two separate feed components which are both gaseous, which requires potentially complex installations such as separate gas lines, valves and a gas mixer. Furthermore, this method relies on the presence of a suitable catalyst for converting the gaseous mixture to HCN. In case of the articles being employed as catalyst the resulting gas composition and HCN level is highly dependent on the surface area and composition of the treated articles in the furnace. This is undesirable in terms of reproducibility and controllability.
GB610953 relates to a process by which nitride cases may be formed on austenitic and stainless steels without the need of a preliminary depassivation (i.e. activation) treatment. The method requires the presence during nitriding of a compound of an alkali or alkaline earth metal with nitrogen or with nitrogen and hydrogen in an atmosphere of a gaseous nitrogen-liberating material, such as ammonia. The alkali or alkaline earth metal compound may be an amide such as sodium amide (NaNH2) or calcium amide (Ca(NH2)2). The alkali or alkaline earth metal compounds are simply heated together with the steel article to a nitriding temperature of 475-600° C. Thus, the compounds are used for forming a case of nitrides in stainless steel. The formation of nitrides is associated with a loss of corrosion resistance.
Hertz et al. (“Technologies for low temperature carburising and nitriding of austenitic stainless steel” INTERNATIONAL HEAT TREATMENT AND SURFACE ENGINEERING, vol. 2, no. 1, 3 Mar. 2008, pages 32-38) discuss carburising and nitriding treatments at low temperatures (350-450° C.), acknowledging the diffusion barrier of oxide layers. The preferred method for activating the article to overcome this diffusion barrier is fluoridation with NF3.
Stock et al. (“Plasma-assisted chemical vapour deposition with titanium amides as precursors” SURFACE AND COATINGS TECHNOLOGY, ELSEVIER, AMSTERDAM, NL, vol. 46, no. 1, 30 May 1991, pages 15-23) relates to the production of wear-resistant coatings such as TiN in low-temperature plasma-assisted chemical vapour deposition. In this regard, it is suggested to use titanium amide (Ti(N(CH3)3)4) together with steel substrates at 200-500° C. to establish such a coating. Stock et al. are silent on any preceding steps for activating the steel surface. Stock et al. exclusively relate to the production of a coating, but are silent on case hardening, i.e. the modification of an existing surface through diffusion treatment.
In view of the mentioned prior art methods, there is still a need for an activation method for a passivated article prior to carburising, nitriding or nitrocarburising, said activation method being simple, energy-efficient and safe.
It is therefore a first object of the present invention to provide a simple and energy-efficient method of activating an article of passive ferrous or non-ferrous metal.
It is a second object of the present invention to provide a safe method of activating an article of passive ferrous or non-ferrous metal, said method minimising health risks.
It is a third object of the present invention to provide a method of activating an article of passive ferrous or non-ferrous metal, which method leads to an improved activation prior to subsequent carburising, nitriding or nitrocarburising.
It is a fourth object of the present invention to provide a method of activating an article of passive ferrous or non-ferrous metal, which method is conveniently coupled with subsequent carburising, nitriding or nitrocarburising.